Inhibition of the SH3 domain-mediated binding of Src to the androgen receptor and its effect on tumor growth

Research resource: EPSLiM: ensemble predictor for short linear motifs in nuclear hormone receptors

Nuclear receptors (NRs) are a superfamily of transcription factors central to regulating many biological processes, including cell growth, death, metabolism, and immune responses. NR-mediated gene expression can be modulated by coactivators and corepressors through direct physical interaction or protein complexes with functional domains in NRs. One class of these domains includes short linear motifs (SLiMs), which facilitate protein-protein interactions, phosphorylation, and ligand binding primarily in the intrinsically disordered regions (IDRs) of proteins. Across all proteins, the number of known SLiMs is limited due to the difficulty in studying IDRs experimentally. Computational tools provide a systematic and data-driven approach for predicting functional motifs that can be used to prioritize experimental efforts. Accordingly, several tools have been developed based on sequence conservation or biophysical features; however, discrepancies in predictions make it difficult to determine the true candidate SLiMs. In this work, we present the ensemble predictor for short linear motifs (EPSLiM), a novel strategy to prioritize the residues that are most likely to be SLiMs in IDRs. EPSLiM applies a generalized linear model to integrate predictions from individual methodologies. We show that EPSLiM outperforms individual predictors, and we apply our method to NRs. The androgen receptor is an example with an N-terminal domain of 559 disordered amino acids that contains several validated SLiMs important for transcriptional activation. We use the androgen receptor to illustrate the predictive performance of EPSLiM and make the results of all human and mouse NRs publically available through the web service http://epslim.bwh.harvard.edu.

Estrogen receptor beta as a novel target of androgen receptor action in breast cancer cell lines

INTRODUCTION

The two isoforms of estrogen receptor (ER) alpha and beta play opposite roles in regulating proliferation and differentiation of breast cancers, with ER-alpha mediating mitogenic effects and ER-beta acting as a tumor suppressor. Emerging data have reported that androgen receptor (AR) activation inhibits ER-positive breast cancer progression mainly by antagonizing ER-alpha signaling. However, to date no studies have specifically evaluated a potential involvement of ER-beta in the inhibitory effects of androgens.

METHODS

ER-beta expression was examined in human breast cancer cell lines using real-time PCR, Western blotting and small interfering RNA (siRNA) assays. Mutagenesis studies, electromobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) analysis were performed to assess the effects of mibolerone/AR on ER-beta promoter activity and binding.

RESULTS

In this study, we demonstrate that mibolerone, a synthetic androgen ligand, up-regulates ER-beta mRNA and protein levels in ER-positive breast cancer cells. Transient transfection experiments, using a vector containing the human ER-beta promoter region, show that mibolerone increases basal ER-beta promoter activity. Site-directed mutagenesis and deletion analysis reveal that an androgen response element (ARE), TGTTCT motif located at positions -383 and -377, is critical for mibolerone-induced ER-beta up-regulation in breast cancer cells. This occurs through an increased recruitment of AR to the ARE site within the ER-beta promoter region, along with an enhanced occupancy of RNA polymerase II. Finally, silencing of ER-beta gene expression by RNA interference is able to partially reverse the effects of mibolerone on cell proliferation, p21 and cyclin D1 expression.

CONCLUSIONS

Collectively, these data provide evidence for a novel mechanism by which activated AR, through an up-regulation of ER-beta gene expression, inhibits breast cancer cell growth.

Targeting androgen receptor/Src complex impairs the aggressive phenotype of human fibrosarcoma cells

Background

Hormones and growth factors influence the proliferation and invasiveness of human mesenchymal tumors. The highly aggressive human fibrosarcoma HT1080 cell line harbors classical androgen receptor (AR) that responds to androgens triggering cell migration in the absence of significant mitogenesis. As occurs in many human cancer cells, HT1080 cells also express epidermal growth factor receptor (EGFR).

Experimental

Findings: We report that the pure anti-androgen Casodex inhibits the growth of HT1080 cell xenografts in immune-depressed mice, revealing a novel role of AR in fibrosarcoma progression. In HT1080 cultured cells EGF, but not androgens, robustly increases DNA synthesis. Casodex abolishes the EGF mitogenic effect, implying a crosstalk between EGFR and AR. The mechanism underlying this crosstalk has been analyzed using an AR-derived small peptide, S1, which prevents AR/Src tyrosine kinase association and androgen-dependent Src activation. Present findings show that in HT1080 cells EGF induces AR/Src Association, and the S1 peptide abolishes both the assembly of this complex and Src activation. The S1 peptide inhibits EGF-stimulated DNA synthesis, cell matrix metalloproteinase-9 (MMP-9) secretion and invasiveness of HT1080 cells. Both Casodex and S1 peptide also prevent DNA synthesis and migration triggered by EGF in various human cancer-derived cells (prostate, breast, colon and pancreas) that express AR.

Conclusion

This study shows that targeting the AR domain involved in AR/Src association impairs EGF signaling in human fibrosarcoma HT1080 cells. The EGF-elicited processes inhibited by the peptide (DNA synthesis, MMP-9 secretion and invasiveness) cooperate in increasing the aggressive phenotype of HT1080 cells. Therefore, AR represents a new potential therapeutic target in human fibrosarcoma, as supported by Casodex inhibition of HT1080 cell xenografts. The extension of these findings in various human cancer-derived cell lines highlights the conservation of this process across divergent cancer cells and identifies new potential targets in the therapeutic approach to human cancers.

Androgen receptors in hormone-dependent and castration-resistant prostate cancer

In the United States, prostate cancer (PCa) is the most commonly diagnosed non-cutaneous cancer in males and the second leading cause of cancer-related death for men. The prostate is an androgen-dependent organ and PCa is an androgen-dependent disease. Androgen action is mediated by the androgen receptor (AR), a hormone activated transcription factor. The primary treatment for metastatic PCa is androgen deprivation therapy (ADT). For the most part, tumors respond to ADT, but most become resistant to therapy within two years. There is persuasive evidence that castration resistant (also termed castration recurrent) PCa (CRPC) remains AR dependent. Recent studies have shown that there are numerous factors that contribute to AR reactivation despite castrate serum levels of androgens. These include changes in AR expression and structure through gene amplification, mutation, and alternative splicing. Changes in steroid metabolism, cell signaling, and coregulator proteins are also important contributors to AR reactivation in CRPC. Most AR targeted therapies have been directed at the hormone binding domain. The finding that constitutively active AR splice variants that lack the hormone binding domain are frequently expressed in CRPC highlights the need to develop therapies that target other portions of AR. In this review, the role of AR in normal prostate, in PCa, and particularly the mechanisms for its reactivation subsequent to ADT are summarized. In addition, recent clinical trials and novel approaches to target AR are discussed.

Fulvestrant-induced cell death and proteasomal degradation of estrogen receptor α protein in MCF-7 cells require the CSK c-Src tyrosine kinase

Fulvestrant is a representative pure antiestrogen and a Selective Estrogen Receptor Down-regulator (SERD). In contrast to the Selective Estrogen Receptor Modulators (SERMs) such as 4-hydroxytamoxifen that bind to estrogen receptor α (ERα) as antagonists or partial agonists, fulvestrant causes proteasomal degradation of ERα protein, shutting down the estrogen signaling to induce proliferation arrest and apoptosis of estrogen-dependent breast cancer cells. We performed genome-wide RNAi knockdown screenings for protein kinases required for fulvestrant-induced apoptosis of the MCF-7 estrogen-dependent human breast caner cells and identified the c-Src tyrosine kinase (CSK), a negative regulator of the oncoprotein c-Src and related protein tyrosine kinases, as one of the necessary molecules. Whereas RNAi knockdown of CSK in MCF-7 cells by shRNA-expressing lentiviruses strongly suppressed fulvestrant-induced cell death, CSK knockdown did not affect cytocidal actions of 4-hydroxytamoxifen or paclitaxel, a chemotherapeutic agent. In the absence of CSK, fulvestrant-induced proteasomal degradation of ERα protein was suppressed in both MCF-7 and T47D estrogen-dependent breast cancer cells whereas the TP53-mutated T47D cells were resistant to the cytocidal action of fulvestrant in the presence or absence of CSK. MCF-7 cell sensitivities to fulvestrant-induced cell death or ERα protein degradation was not affected by small-molecular-weight inhibitors of the tyrosine kinase activity of c-Src, suggesting possible involvement of other signaling molecules in CSK-dependent MCF-7 cell death induced by fulvestrant. Our observations suggest the importance of CSK in the determination of cellular sensitivity to the cytocidal action of fulvestrant.

Small-molecule hormones: molecular mechanisms of action

Small-molecule hormones play crucial roles in the development and in the maintenance of an adult mammalian organism. On the molecular level, they regulate a plethora of biological pathways. Part of their actions depends on their transcription-regulating properties, exerted by highly specific nuclear receptors which are hormone-dependent transcription factors. Nuclear hormone receptors interact with coactivators, corepressors, basal transcription factors, and other transcription factors in order to modulate the activity of target genes in a manner that is dependent on tissue, age and developmental and pathophysiological states. The biological effect of this mechanism becomes apparent not earlier than 30-60 minutes after hormonal stimulus. In addition, small-molecule hormones modify the function of the cell by a number of nongenomic mechanisms, involving interaction with proteins localized in the plasma membrane, in the cytoplasm, as well as with proteins localized in other cellular membranes and in nonnuclear cellular compartments. The identity of such proteins is still under investigation; however, it seems that extranuclear fractions of nuclear hormone receptors commonly serve this function. A direct interaction of small-molecule hormones with membrane phospholipids and with mRNA is also postulated. In these mechanisms, the reaction to hormonal stimulus appears within seconds or minutes.

A mechanistic study of the effect of doxorubicin/adriamycin on the estrogen response in a breast cancer model

OBJECTIVE

Estrogen treatment limits the cytotoxic effects of chemotherapy in estrogen receptor-positive (ER+) breast cancer cell lines, suggesting that estrogen pathway signaling may confer chemotherapeutic resistance. This study investigates the molecular responses of ER+ breast cancer cell lines to the chemotherapeutic agent, doxorubicin, in the presence or absence of estrogen.

METHODS

ER+ MCF-7 and T47-D cells were cultured in hormone-starved or estrogen-containing media with or without doxorubicin at concentrations mimicking the low concentrations seen in plasma and tumor microenvironments in humans following typical bolus administration. Protein levels, phosphorylations, and interactions of estrogen-signaling molecules were assessed following these treatments, as well the effects of ER signaling inhibitors on cell proliferation.

RESULTS

Surprisingly, estrogen and doxorubicin co-treatment markedly induced pro-growth alterations compared to doxorubicin alone and modestly enhanced estrogen alone-induced changes. Several inhibitors suppressed cell proliferation in the presence of doxorubicin and estrogen.

CONCLUSIONS

These findings demonstrate that molecular changes caused by doxorubicin in ER+ breast cancer cells can be reversed by estrogen, providing molecular evidence for the poorer responses of ER+ tumors to doxorubicin in the presence of physiologic estrogen levels. Our results also suggest that the addition of drugs targeting the ER, EGFR, the SFKs, MEK, PI3K, and/or the MMP proteins to a conventional chemotherapy regimen may improve chemosensitivity.

Polyproline and Tat transduction peptides in the study of the rapid actions of steroid receptors

Cellular responses to signals require the action of a myriad of protein networks, which are regulated by protein/protein associations. Rapid actions of steroid hormones are also subject to this regulation. They induce direct association of steroid receptors with different proteins (e.g., growth factor receptors, signaling effectors, scaffold proteins, transcription factors). These multi-molecular complexes drive signaling activation and finally trigger basic hormonal effects. Receptor/protein associations are attracting increased interest concerning their role in hormone action as well as their potential use as therapeutic targets in hormonal diseases.

Caveolin-1 and prostate cancer progression

Caveolin-1 was identified in the 1990s as a marker of aggressive prostate cancer. The caveolin-1 protein localizes to vesicular structures called caveolae and has been shown to bind and regulate many signaling proteins involved in oncogenesis. Caveolin-1 also has lipid binding properties and mediates aspects of cholesterol and fatty acid metabolism and can elicit biological responses in a paracrine manner when secreted. Caveolin-1 is also present in the serum of prostate cancer patients and circulating levels correlate with extent of disease. Current evidence indicates that increased expression of caveolin-1 in prostate adenocarcinoma cells and commensurate downregulation of the protein in prostate stroma, mediate progression to the castration-resistant phase of prostate cancer through diverse pathways. This chapter summarizes the current state of our understanding of the cellular and physiologic mechanisms in which caveolin-1 participates in the evolution of prostate cancer cell phenotypes.

Minireview: Recent advances in extranuclear steroid receptor actions

The participation of extranuclear steroid receptor signaling in organ physiology and the impact for pathobiology has increasingly been demonstrated. Important functions of membrane estrogen receptors in the cardiovascular system demonstrate new mechanisms of rapid steroid signaling to gene regulation, preventing cardiovascular disease and maintaining healthy arterial function. In cancer cells, kinase signaling initiated by extranuclear estrogen, progesterone, and androgen receptors modulates transcriptional events in the nucleus, which in turn regulate proliferation, migration, and invasion. Important mediators of cross talk between cytoplasmic and nuclear steroid receptor signaling are the proline-, glutamic acid-, and leucine-rich protein-1 and paxillin proteins, both of which modulate membrane and nuclear receptor pool signaling to promote a variety of cell biological functions.

Integrins regulate microtubule nucleating activity of centrosome through mitogen-activated protein kinase/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase (MEK/ERK) signaling

Microtubule nucleation is an essential step in the formation of the microtubule cytoskeleton. We recently showed that androgen and Src promote microtubule nucleation and γ-tubulin accumulation at the centrosome. Here, we explore the mechanisms by which androgen and Src regulate these processes and ask whether integrins play a role. We perturb integrin function by a tyrosine-to-alanine substitution in membrane-proximal NPIY motif in the integrin β1 tail and show that this mutant substantially decreases microtubule nucleation and γ-tubulin accumulation at the centrosome. Because androgen stimulation promotes the interaction of the androgen receptor with Src, resulting in PI3K/AKT and MEK/ERK signaling, we asked whether these pathways are inhibited by the mutant integrin and whether they regulate microtubule nucleation. Our results indicate that the formation of the androgen receptor-Src complex and the activation of downstream pathways are significantly suppressed when cells are adhered by the mutant integrin. Inhibitor studies indicate that microtubule nucleation requires MEK/ERK but not PI3K/AKT signaling. Importantly, the expression of activated RAF-1 is sufficient to rescue microtubule nucleation inhibited by the mutant integrin by promoting the centrosomal accumulation of γ-tubulin. Our data define a novel paradigm of integrin signaling, where integrins regulate microtubule nucleation by promoting the formation of androgen receptor-Src signaling complexes to activate the MEK/ERK signaling pathway.

Analysis of androgen receptor rapid actions in cellular signaling pathways: receptor/Src association

Much evidence indicates that, with few exceptions, non-genomic actions of steroids are mediated by receptors universally known as nuclear receptors. Steroid receptors do not exhibit intrinsic tyrosine kinase activity. Nevertheless, they stimulate different signaling pathways in cytoplasm of target cells, including those dependent on Src, a cytoplasmic tyrosine kinase. Steroid-induced Src activation regulates cell cycle progression, survival, migration, and associated processes, such as cell growth and differentiation. Androgen stimulation of human prostate cancer-derived LNCaP cells triggers cell cycle progression and proliferation. The key event in this process is the association of androgen receptor (AR) with Src. This association triggers activation of the Src/Ras/Erk pathway and finally impacts cell cycle. Androgen stimulation of fibroblasts also induces AR/Src association, which triggers DNA synthesis. Prevention of this association by a receptor-derived peptide competing for AR interaction with Src specifically inhibits the androgen receptor-dependent proliferative effect in vitro and in vivo.

Understanding extranuclear (nongenomic) androgen signaling: what a frog oocyte can tell us about human biology

Steroids are key factors in a myriad of mammalian biological systems, including the brain, kidney, heart, bones, and gonads. While alternative potential steroid receptors have been described, the majority of biologically relevant steroid responses appear to be mediated by classical steroid receptors that are located in all parts of the cell, from the plasma membrane to the nucleus. Interestingly, these classical steroid receptors modulate different signals depending upon their location. For example, receptors in the plasma membrane interact with membrane signaling molecules, including G proteins and kinases. In contrast, receptors in the nucleus interact with nuclear signaling molecules, including transcriptional co-regulators. These extranuclear and intranuclear signals function together in an integrated fashion to regulate important biological functions. While most studies on extranuclear steroid signaling have focused on estrogens, recent work has demonstrated that nongenomic androgen signaling is equally important and that these two steroids modulate similar signaling pathways. In fact, by taking advantage of a simple model system whereby a physiologically relevant androgen-mediated process is regulated completely independent of transcription (Xenopus laevis oocyte maturation), many novel and conserved concepts in nongenomic steroid signaling have been uncovered and characterized.

Raloxifene induces cell death and inhibits proliferation through multiple signaling pathways in prostate cancer cells expressing different levels of estrogen receptor α and β

Raloxifene (RAL), a selective estrogen receptor (ER) modulator (SERM) seems to induce apoptosis in both androgen-dependent and -independent prostate cell (PC) lines via activation of ERβ and an antagonistic effect on ERα. In this study, we evaluated the effects of RAL on epithelial PC growth using the two following in vitro models: the androgen-dependent cell line EPN which expressed both ERs; and a stabilized epithelial cell line derived from a prostate cancer specimen (CPEC), which expressed low levels of ERβ and lacked ERα. In EPN cells, there was an increase in the pre-G1 apoptotic peak and a reduction in the S phase of the cell cycle with G0/G1 arrest after E2 or RAL treatment; bcl-2 mRNA and Bcl-2 protein levels were significantly reduced, while activated caspase-3 and Par-4 levels increased significantly after either E2 or RAL treatment; in addition, c-myc transcript was inhibited after 10(-6)  M RAL treatment. A dose-dependent increase of metallothionein II gene RNA level was also induced by RAL in EPN. In CPEC, there was only a weak apoptotic peak associated with caspase-3 activation and Par-4 increase after either E2 or RAL treatment; while c-myc transcript level increased. RAL induced a rapid but transient phosphorylation of ERK 1/2 in EPN cells but generated a sustained effect in CPEC. These findings suggest that RAL effects on PC growth control in vitro are cell-specific, depending on ERβ or ERβ/ERα relative expression levels. Moreover, this study demonstrated that RAL affected both transcriptional regulation and non-genomic signals, which resulted in the modulation of multiple signaling pathways of apoptosis and of cell cycle progression.

Androgen-induced cell migration: role of androgen receptor/filamin A association

BACKGROUND

Androgen receptor (AR) controls male morphogenesis, gametogenesis and prostate growth as well as development of prostate cancer. These findings support a role for AR in cell migration and invasiveness. However, the molecular mechanism involved in AR-mediated cell migration still remains elusive.

METHODOLOGY/PRINCIPAL FINDINGS

Mouse embryo NIH3T3 fibroblasts and highly metastatic human fibrosarcoma HT1080 cells harbor low levels of transcriptionally incompetent AR. We now report that, through extra nuclear action, AR triggers migration of both cell types upon stimulation with physiological concentrations of the androgen R1881. We analyzed the initial events leading to androgen-induced cell migration and observed that challenging NIH3T3 cells with 10 nM R1881 rapidly induces interaction of AR with filamin A (FlnA) at cytoskeleton. AR/FlnA complex recruits integrin beta 1, thus activating its dependent cascade. Silencing of AR, FlnA and integrin beta 1 shows that this ternary complex controls focal adhesion kinase (FAK), paxillin and Rac, thereby driving cell migration. FAK-null fibroblasts migrate poorly and Rac inhibition by EHT impairs motility of androgen-treated NIH3T3 cells. Interestingly, FAK and Rac activation by androgens are independent of each other. Findings in human fibrosarcoma HT1080 cells strengthen the role of Rac in androgen signaling. The Rac inhibitor significantly impairs androgen-induced migration in these cells. A mutant AR, deleted of the sequence interacting with FlnA, fails to mediate FAK activation and paxillin tyrosine phosphorylation in androgen-stimulated cells, further reinforcing the role of AR/FlnA interaction in androgen-mediated motility.

CONCLUSIONS/SIGNIFICANCE

The present report, for the first time, indicates that the extra nuclear AR/FlnA/integrin beta 1 complex is the key by which androgen activates signaling leading to cell migration. Assembly of this ternary complex may control organ development and prostate cancer metastasis.

Regulation of Sertoli-germ cell adhesion and sperm release by FSH and nonclassical testosterone signaling

Testosterone and FSH act in synergy to produce the factors required to maximize the production of spermatozoa and male fertility. However, the molecular mechanisms by which these hormones support spermatogenesis are not well established. Recently, we identified a nonclassical mechanism of testosterone signaling in cultured rat Sertoli cells. We found that testosterone binding to the androgen receptor recruits and activates Src tyrosine kinase. Src then causes the activation of the epidermal growth factor receptor, which results in the phosphorylation and activation of the ERK MAPK and the cAMP response element-binding protein transcription factor. In this report, we find that FSH inhibits testosterone-mediated activation of ERK and the MAPK pathway in Sertoli cells via the protein kinase A-mediated inhibition of Raf kinase. In addition, FSH, as well as inhibitors of Src and ERK kinase activity, reduced germ cell attachment to Sertoli cells in culture. Using pathway-specific androgen receptor mutants we found that the nonclassical pathway is required for testosterone-mediated increases in germ cell attachment to Sertoli cells. Studies of seminiferous tubule explants determined that Src kinase, but not ERK kinase, activity is required for the release of sperm from seminiferous tubule explants. These findings suggest the nonclassical testosterone-signaling pathway acts via Src and ERK kinases to facilitate the adhesion of immature germ cells to Sertoli cells and through Src to permit the release of mature spermatozoa. In contrast, FSH acts to limit testosterone-mediated ERK kinase activity and germ cell attachment.

Steroid signaling activation and intracellular localization of sex steroid receptors

In addition to stimulating gene transcription, sex steroids trigger rapid, non-genomic responses in the extra-nuclear compartment of target cells. These events take place within seconds or minutes after hormone administration and do not require transcriptional activity of sex steroid receptors. Depending on cell systems, activation of extra-nuclear signaling pathways by sex steroids fosters cell cycle progression, prevents apoptosis, leads to epigenetic modifications and increases cell migration through cytoskeleton changes. These findings have raised the question of intracellular localization of sex steroid receptors mediating these responses. During the past years, increasing evidence has shown that classical sex steroid receptors localized in the extra-nuclear compartment or close to membranes of target cells induce these events. The emerging picture is that a process of bidirectional control between signaling activation and sex steroid receptor localization regulates the outcome of hormonal responses in target cells. This mechanism ensures cell cycle progression in estradiol-treated breast cancer cells, and its derangement might occur in progression of human proliferative diseases. These findings will be reviewed here together with unexpected examples of the relationship between sex steroid receptor localization, signaling activation and biological responses in target cells. We apologize to scientists whose reports are not mentioned or extensively discussed owing to space limitations.

Peroxisome proliferator-activated receptor gamma (PPARgamma): Is the genomic activity the only answer?

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear hormone receptor superfamily of transcription factors, widely expressed in the organism, including adipose, vascular and immune cells. Besides the well-known role in lipid/glycidic homeostasis, PPARgamma has also recently emerged as a key regulator of inflammatory and immune responses. Besides the natural ligands, more potent synthetic agonists of PPARgamma have been developed, including thiazolidinediones (TZDs), currently used in type 2 diabetes treatment, which also exert anti-inflammatory and anti-neoplastic effects. PPARgamma mechanism of action has focused considerable attention over the years. This receptor was initially shown to act on gene expression through a direct transcription and an indirect transrepression activity, mainly associated with metabolic and anti-inflammatory effects. Different post-translational modifications of the receptor can modulate PPARgamma activity. More recently, rapid nongenomic activity of TZDs affecting post-translation modifications of extranuclear proteins involved in cell signaling, has been reported. In particular, PPARgamma can physically interact with protein kinases resulting in a compartment specific recruitment and activity modulation of these enzymes. Among them, ERK can be positively/negatively regulated by PPARgamma ligands, as in endothelial cells, where TZDs exert anti-inflammatory effects through a novel mechanism involving a rapid inhibition of ERK1/2 phosphorylation/activation. Finally, some of the TZD anti-tumor effects seem to be PPARgamma-independent, raising the possibility that alternative receptors can act through extranuclear nongenomic pathways. In conclusion, different mechanisms of action of PPARgamma seem to coexist in an interacting functional network in the cell, concurring in mediating both pharmacological and natural ligand effects.

SRC kinase inhibition: targeting bone metastases and tumor growth in prostate and breast cancer

Prostate and breast cancer cells preferentially metastasize to bone, whereupon a complex interaction between metastatic tumor cells, osteoclasts, and osteoblasts results in the development of bone lesions that cause significant pain and patient morbidity. For patients with bone lesions, the goals of treatment are to decrease tumor growth, prevent further metastases, and inhibit tumor-associated bone pathology. Preclinical data suggest that SRC, a nonreceptor tyrosine kinase, is an important signaling molecule during the processes of osteoclast-mediated bone resorption, tumor growth, and metastasis, and that SRC has a role in hormone receptor signaling and resistance. As such, SRC represents a logical target for the treatment of advanced metastatic prostate or breast cancer. SRC-targeting agents, including dasatinib, saracatinib, and bosutinib, are currently in clinical development for patients with solid tumors. Preliminary data from phase 1/2 trials, including tumor responses and bone-specific activity in patients with prostate or breast cancer, demonstrate that SRC inhibitors have potential in the clinical setting. Data arising from ongoing and future clinical trials will confirm whether SRC inhibitors provide clinical benefits for patients with advanced disease.

The neuroendocrine-derived peptide parathyroid hormone-related protein promotes prostate cancer cell growth by stabilizing the androgen receptor

During progression to an androgen-independent state following androgen ablation therapy, prostate cancer cells continue to express the androgen receptor (AR) and androgen-regulated genes, indicating that AR is critical for the proliferation of hormone-refractory prostate cancer cells. Multiple mechanisms have been proposed for the development of AR-dependent hormone-refractory disease, including changes in expression of AR coregulatory proteins, AR mutation, growth factor-mediated activation of AR, and AR protein up-regulation. The most prominent of these progressive changes is the up-regulation of AR that occurs in >90% of prostate cancers. A common feature of the most aggressive hormone-refractory prostate cancers is the accumulation of cells with neuroendocrine characteristics that produce paracrine factors and may provide a novel mechanism for the regulation of AR during advanced stages of the disease. In this study, we show that neuroendocrine-derived parathyroid hormone-related protein (PTHrP)-mediated signaling through the epidermal growth factor receptor (EGFR) and Src pathways contributes to the phenotype of advanced prostate cancer by reducing AR protein turnover. PTHrP-induced accumulation of AR depended on the activity of Src and EGFR and consequent phosphorylation of the AR on Tyr(534). PTHrP-induced tyrosine phosphorylation of AR resulted in reduced AR ubiquitination and interaction with the ubiquitin ligase COOH terminus of Hsp70-interacting protein. These events result in increased accumulation of AR and thus enhanced growth of prostate cancer cells at low levels of androgen.

SRC family kinase activity is up-regulated in hormone-refractory prostate cancer

PURPOSE

Although Src family kinase (SFK) inhibitors are now in clinical trials for the treatment of androgen-independent prostate cancer (AIPC), there are no studies relating SFK activation to patient survival. This study was designed to determine if SFK activation was up-regulated with the development of AIPC and if patients could be selected who were more likely to respond to therapy.

EXPERIMENTAL DESIGN

A unique cohort of matched prostate tumor samples, taken before hormone deprivation therapy and following hormone relapse, was used to determine by immunohistochemistry on an individual patient basis if SFK activity changed with progression to AIPC and whether this related to patient outcome measures. Using matched, hormone-sensitive and hormone-refractory cell lines, we determined if hormone status affected the way prostate cancer cells respond to suppression of SFK activity by the small-molecule inhibitor dasatinib.

RESULTS

In the current study, 28% of patients with AIPC exhibited an increase in SFK activity in prostate cancer tissue, these patients had significantly shorter overall survival (P<0.0001), and activated SFK expression correlated with the presence of distant metastases. Dasatinib inhibited phosphorylation of Src and Lyn and the downstream substrate FAK in hormone-sensitive and hormone-refractory cell lines. Although migration was reduced by dasatinib in both cell lines, proliferation of hormone-refractory cells only was inhibited.

CONCLUSION

Appropriate patient selection may allow better targeting of prostate cancer patients who are likely to respond to the treatment with SFK inhibitors at the same time improving the outcome of clinical trials.

Lef1 haploinsufficient mice display a low turnover and low bone mass phenotype in a gender- and age-specific manner

We investigated the role of Lef1, one of the four transcription factors that transmit Wnt signaling to the genome, in the regulation of bone mass. Microcomputed tomographic analysis of 13- and 17-week-old mice revealed significantly reduced trabecular bone mass in Lef1(+/-) females compared to littermate wild-type females. This was attributable to decreased osteoblast activity and bone formation as indicated by histomorphometric analysis of bone remodeling. In contrast to females, bone mass was unaffected by Lef1 haploinsufficiency in males. Similarly, females were substantially more responsive than males to haploinsufficiency in Gsk3beta, a negative regulator of the Wnt pathway, displaying in this case a high bone mass phenotype. Lef1 haploinsufficiency also led to low bone mass in males lacking functional androgen receptor (AR) (tfm mutants). The protective skeletal effect of AR against Wnt-related low bone mass is not necessarily a result of direct interaction between the AR and Wnt signaling pathways, because Lef1(+/-) female mice had normal bone mass at the age of 34 weeks. Thus, our results indicate an age- and gender-dependent role for Lef1 in regulating bone formation and bone mass in vivo. The resistance to Lef1 haploinsufficiency in males with active AR and in old females could be due to the reduced bone turnover in these mice.

Targeting Src signaling in metastatic bone disease

Src is a tyrosine kinase involved in the regulation of a range of cellular processes including proliferation, adhesion, motility and survival. In addition, it is a key regulator of bone metabolism. Src has been implicated in the pathogenesis of a number of cancers, and has been found to be overexpressed in breast, prostate, colorectal, pancreatic and nonsmall-cell lung tumors. There is also evidence that aberrant Src signaling may contribute to the increased osteoclastic activity associated with bone metastases. Bone metastases frequently occur in cancer patients with advanced disease. The metastasized cells disrupt normal bone remodeling pathways resulting in the release of growth factors that further promote tumor growth. Thus, a cycle of metastatic bone destruction is initiated, leading to compromised skeletal integrity and substantially reduced quality of life. Because of the role of Src in both cancer development and in bone metabolism, it may provide a therapeutic target for patients with bone metastases.

Steroidal androgens and nonsteroidal, tissue-selective androgen receptor modulator, S-22, regulate androgen receptor function through distinct genomic and nongenomic signaling pathways

Androgen receptor (AR) ligands are important for the development and function of several tissues and organs. However, the poor oral bioavailability, pharmacokinetic properties, and receptor cross-reactivity of testosterone, coupled with side effects, place limits on its clinical use. Selective AR modulators (SARMs) elicit anabolic effects in muscle and bone, sparing reproductive organs like the prostate. However, molecular mechanisms underlying the tissue selectivity remain ambiguous. We performed a variety of in vitro studies to compare and define the molecular mechanisms of an aryl propionamide SARM, S-22, as compared with dihydrotestosterone (DHT). Studies indicated that S-22 increased levator ani muscle weight but decreased the size of prostate in rats. Analysis of the upstream intracellular signaling events indicated that S-22 and DHT mediated their actions through distinct pathways. Modulation of these pathways altered the recruitment of AR and its cofactors to the PSA enhancer in a ligand-dependent fashion. In addition, S-22 induced Xenopus laevis oocyte maturation and rapid phosphorylation of several kinases, through pathways distinct from steroids. These studies reveal novel differences in the molecular mechanisms by which S-22, a nonsteroidal SARM, and DHT mediate their pharmacological effects.

ValiRx plc

ValiRx plc is a therapeutics and diagnostics company developing an integrated approach to the diagnosis, treatment and prognosis of cancer through its two subsidiaries; ValiPharma and ValiBio. Over 95% of cellular DNA is tightly packaged into a complex structure called chromatin, with only 1% available to be read by cell's machinery. ValiRx's two proprietary technology platforms exploit this epigenomic structure. ValiBio is developing low-cost, rapid, high-throughput, noninvasive screening tests for the early detection, differential diagnosis and prognosis of cancer using its patented Hypergenomics™ and Nucleosomics™ technology. Its therapeutics subsidiary, ValiPharma, is developing novel gene-silencing therapeutics based on its GeneICE™ technology platform, which works by repackaging specific open areas of DNA, resulting in targeted gene deactivation. HyperGenomics and GeneICE are synergistic but independent business areas based on the company's core patent portfolio. ValiRx intends to facilitate early, optimal personalized treatment regimes by correlating 'hypersensitive' site profiles within the genome to specific types of cancer.

Targeting the androgen receptor pathway in prostate cancer

When prostate cancers progress following androgen depletion therapy, there are currently few treatment options with only one, docetaxel, that has been shown to prolong life. Recent work has shown that castration-resistant prostate cancers (CRPCs) continue to depend on androgen receptor (AR) signaling which is reactivated despite low serum androgen levels. Currently available AR-targeted therapy, including GnRH agonists and antiandrogens, cannot completely shut down AR signaling. Several mechanisms that enhance AR signaling in an androgen-depleted environment have been elucidated. These include AR mutations that allow activation by low androgen levels or by other endogenous steroids, AR overexpression, increased local intracrine synthesis of androgens, and upregulation of tyrosine kinase pathways. This has led to the development of a number of novel agents targeting the AR signaling pathway, including more effective antiandrogens, inhibitors of CYP17, an enzyme required for androgen synthesis, inhibitors of 5alpha-reductase, inhibitors of HSP90 which protects AR from degradation, inhibitors of histone deacetylases which is required for optimal AR-mediated transcription, as well as inhibitors of tyrosine kinase inhibitors. Many of these strategies are currently being tested in clinical trials in CRPC.

Hormone-dependent nuclear export of estradiol receptor and DNA synthesis in breast cancer cells

In breast cancer cells, cytoplasmic localization of the estradiol receptor alpha (ERalpha) regulates estradiol-dependent S phase entry. We identified a nuclear export sequence (NES) in ERalpha and show that its export is dependent on both estradiol-mediated phosphatidylinositol-3-kinase (PI3K)/AKT activation and chromosome region maintenance 1 (CRM1). A Tat peptide containing the ERalpha NES disrupts ERalpha-CRM1 interaction and prevents nuclear export of ERalpha- and estradiol-induced DNA synthesis. NES-ERalpha mutants do not exit the nucleus and inhibit estradiol-induced S phase entry; ERalpha-dependent transcription is normal. ERalpha is associated with Forkhead proteins in the nucleus, and estradiol stimulates nuclear exit of both proteins. ERalpha knockdown or ERalpha NES mutations prevent ERalpha and Forkhead nuclear export. A mutant of forkhead in rhabdomyosarcoma (FKHR), which cannot be phosphorylated by estradiol-activated AKT, does not associate with ERalpha and is trapped in the nucleus, blocking S phase entry. In conclusion, estradiol-induced AKT-dependent phosphorylation of FKHR drives its association with ERalpha, thereby triggering complex export from the nucleus necessary for initiation of DNA synthesis and S phase entry.

Androgens transduce the G alphas-mediated activation of protein kinase A in prostate cells

Androgens regulate the development and function of male reproductive organs and play a crucial role in the onset and progression of prostate cancer. Androgen action is primarily mediated through the nuclear androgen receptor (AR) which acts as a ligand-dependent transcription factor. This mode of androgen action takes hours to manifest and is called the genomic pathway. The androgen-mediated genomic responses require activity of cyclic AMP (cAMP)-dependent protein kinase (PKA). Androgens also act through nongenomic pathways in certain cell types to evoke rapid responses (manifested in minutes) that are mediated through changes in ion currents and second messengers. Here, we show that androgen causes the rapid and cAMP-dependent activation of PKA in prostate cells. The androgen-induced PKA activation is not inhibited by nuclear AR antagonist bicalutamide and can be observed in cells that do not express nuclear AR gene. Reduction of G alphas expression with siRNA attenuates the androgen-mediated activation of PKA, which is required for the androgen-induced prostate cell proliferation. We conclude that androgen actively evokes a nongenomic signaling pathway to activate PKA that is needed for the genomic functioning of nuclear AR. The inhibition of PKA activation, together with standard AR-targeted therapies, may be more efficacious for treatment of patients with prostate cancer.